Metabolic syndrome affects 25% of all Veteran's patients. Chemerin, a chemoattractant and an adipokine, and its receptor CMKLR1, are important in adipogenesis and adipocyte metabolism. CMKLR1-/- mice develop glucose intolerance. Elevated serum chemerin is associated with type 2 diabetes (T2D), and a SNP in the chemerin gene correlates with an increased risk of T2D. Chemerin is secreted as an inactive precursor, and its activation and inactivation is dependent on specific proteolytic cleavages at its C-terminus. We have developed specific ELISAs for the different human and mouse chemerin isoforms. Activated complement C3a and C5a are important in the development of insulin resistance (IR). Carboxypeptidase B2 (CPB2) regulates in vivo complement activity by inactivating both C3a and C5a. CPB2 also activates chemerin. We have utilized CPB2-/- mice in different mouse models of inflammatory disease. We hypothesize that activation of both complement and chemerin are critical in the development of IR and metabolic syndrome in mice and humans. We speculate that the absence of CPB2 will result in increased activation of complement and the development of IR. Specific Aim 1. Determine the levels of C3a, C5a and the different chemerin isoforms in adipose tissues and peripheral blood in a mouse model of obesity. Our preliminary studies show that HFD-fed obese mice have increased levels of chemerin 156S, the most active form of chemerin in mice, in the plasma and subcutaneous adipose tissues. We will examine the time course of complement and chemerin activation over a 24-week HFD-induced obesity study, followed by calorie restriction, in relationship to other inflammatory biomarkers. We will also assess if there is crosstalk between complement and chemerin activation in adipocytes. Specific Aim 2. Determine the levels of C3a, C5a and the different chemerin isoforms in adipose tissues and peripheral blood in humans. 2.1 Obese patients undergoing bariatric surgery and 6-month post-surgery. Our preliminary studies show that patients undergoing bariatric surgery have elevated levels of chem157S, the most active form of chemerin in humans, in the plasma and omental and subcutaneous adipose tissues. They also have elevated C5a levels in adipose tissue. We will examine the levels of complement and different chemerin isoforms in the plasma and adipose tissue of 100 patients undergoing bariatric surgery and 50 nondiabetic control patients. We will correlate these levels with other cytokines and adipokines. 2.2 Assess the potential of chemerin isoforms and complement as a biomarker for IR. Plasma samples will be obtained from patients who are either insulin-resistant (obese or lean), or insulin- sensitive (obese or lean), whose insulin sensitivity or resistance status had been rigorously defined. We will determine the levels of C3a, C5a and chemerin isoforms in 25 samples from each of the four groups to assess whether these parameters can act as a biomarker for IR. Specific Aim 3. Determine if the CPB2-/- mice are predisposed to the development of HFD-induced IR. To test whether CPB2- /- mice are more susceptible to IR, WT and CPB2-/- mice will be placed on HFD or low-fat diet for 8- weeks. IR will be assessed by glucose and insulin intolerance, and hepatic steatosis. If CPB2-/- mice are predisposed to IR, we will attempt to reverse the phenotype using either a C3a or C5a receptor antagonist. Levels of different chemerin isoforms will also be determined. We are the only group that has specific chemerin ELISAs, which permits study of in vivo changes of chemerin levels in humans and mice. This will be the first study to define the role of CPB2 in IR. These studies will provide new information on the activation of complement and chemerin in IR, and may lead to innovative diagnostic tools for the risk assessment of metabolic syndrome and the identification of new therapeutic targets.